AD6688 datasheet(28/138 Pages) AD

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AD6688

Data Sheet

Rev. 0 | Page 28 of 138

Analog Input Buffer Controls and SFDR Optimization

VCM

BUFFER

VIN+

VIN–

AVDD3

100Ω

AVDD3

0.3pF

REG

(0x0008,

0x1908)

REG (0x0008, 0x1A4C,

0x1A4D, 0x1910)

AVDD3

Figure 48. Analog Input Controls

The AD6688 input buffer offers flexible controls for the analog

inputs, such as buffer current, dc coupling, and input full-scale

adjustment. All the available controls are shown in Figure 48.

Using the 0x1A4C and 0x1A4D registers, the buffer behavior on

each channel can be adjusted to optimize the SFDR over various

input frequencies and bandwidths of interest. Register 0x1910 can

be used to change the internal reference voltage. Changing the

internal reference voltage results in a change in the input full-

scale voltage.

When the input buffer current in Register 0x1A4C and

AVDD3 supply changes. This relationship is shown in Figure 49.

For a complete list of buffer current settings, see Table 46.

0.17

0.18

0.19

0.2

0.21

0.22

0.23

0.24

0.25

0.26

400

500

600

700

BUFFER CURRENT SETTING (µA)

Figure 49. AVDD3 Current (IAVDD3) vs. Buffer Control 1 Setting in

Table 10 shows the recommended values for the buffer current

for various Nyquist zones.

Table 10. SFDR Optimization for Input Frequencies

Product

Frequency

0x1A4C and 0x1A4D

AD6688

DC to 1500 MHz

400 µA/500 µA

1500 MHz to 3000 MHz

500 µA

>3000 MHz

500 µA/700 µA

Dither

The AD6688 has internal on-chip dither circuitry that improves

the ADC linearity and SFDR, particularly at smaller signal

levels. A known but random amount of white noise is injected

into the input of the AD6688. This dither improves the small

signal linearity within the ADC transfer function and is

precisely subtracted out digitally. The dither is turned on by

default and does not reduce the ADC input dynamic range. The

data sheet specifications and limits are obtained with the dither

turned on.

The dither is on by default. It is not recommended to turn it off.

Absolute Maximum Input Swing

The absolute maximum input swing allowed at the inputs of the

AD6688 is 5.8 V p-p differential. Signals operating near or at

this level can cause permanent damage to the ADC. See Table 6

for more information.

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